The invention relates to an artificial intervertebral disc in accordance with the preamble of claim 1.
The intervertebral disc takes on a plurality of central functions simultaneously in the vertebral column. It functions as a damper, as a spacer body and also as a joint between the vertebral bodies. The demands to be made on an implant which is intended to serve as a replacement for a natural intervertebral disc in its function as an artificial intervertebral disc are correspondingly complex. For instance, the artificial intervertebral disc must naturally be made up of biocompatible materials and, as a permanent implant, must fulfill its function for the patient for life where possible. In addition to the simple function as a spacer body, the artificial intervertebral disc must in particular be able to effectively cushion the impact forces occurring in the vertebral column so that the vertebrae are not subject to excess stressxe2x80x94however, without noticeably hindering the movability of the vertebrae. A fixed connection must be ensured between the artificial intervertebral disc and the adjoining vertebra in order to suitably lead off the natural stresses of turning, tilting and shearing such as typically occur in the vertebral column.
The highest demands are thus also to be made, in particular on the elastic properties of the artificial intervertebral disc, both as regards its behaviour with respect to torsion stresses, flexural stresses and shear stresses and with respect to pressure stresses. Overall, this means that the mechanical properties of the artificial intervertebral disc have to be reproduced as identically as possible to those of the natural intervertebral disc.
Numerous approaches are known for the replication of the natural properties of an intervertebral disc. For instance, artificial intervertebral discs are known from EP-0 346 269 B1 which consist of two end plates, which are oppositely arranged, are connected via a corrugated tube and are filled with a viscoelastic material. The hitherto unsolved problem of such arrangements, however, also lies in reproducing the suitable, that is the natural, very non-linear, characteristics of an intervertebral disc in the usual stress range for pressure stresses, tensions stresses, shear stresses, bending stresses, flexural stresses and torsion stresses in an artificial intervertebral disc.
It is therefore the object of the invention to propose an artificial intervertebral disc which reproduces the complex, elastic properties of a natural intervertebral disc as exactly as possible.
The artificial intervertebral disc which satisfies this object is characterised by the features of independent claim 1. The dependent claims relate to particularly advantageous embodiments of the invention.
The artificial intervertebral disc of the invention for implantation between two adjacent vertebral bodies comprises two end plates which bound a hollow space, which is filled with an elastically and/or plastically deformable nucleus, at two opposite sides, with the hollow space being enclosed by a tubular fibre ring such that the end plates, together with the fibre ring, bound the hollow space and the end plates are in tensile connection with the fibre ring. The fibre ring is designed as elastically extensible in the radial and axial directions, with the fibre ring having a larger modulus of elasticity than the nucleus. The nucleus and the fibre ring interact such that the elastic properties of the artificial intervertebral disc show a non-linear behaviour with increasing deformation at least with respect to a compression force.
In the artificial intervertebral disc of the invention, two opposite end plates are connected at their peripheries by means of an elastically deformable fibre ring which exerts a constant tensile force on the end plates in the unstressed state such that the nucleus, which is located between the two end plates within the fibre ring, is under pressure bias. An essential disadvantage of known artificial intervertebral discs lies in the fact that under the effect of an elastic deformation either too long a path is stressed until the actual stress region is reached or the starting stiffness already shows values which are too high. A decisive advantage of the artificial intervertebral disc of the invention can therefore be seen in the fact that its elastic behaviour shows a non-linear behaviour at even small deformations at least with respect to compression.
The elastic behaviour of the artificial intervertebral disc of the invention is in this respect determined by the materials encompassing the fibre ring and the nucleus and in particular by the design-determined mechanical coupling of the fibre ring and the nucleus adjoining it, with the specific design of the fibre ring, among other things, being of special importance for the elastic properties of the artificial intervertebral disc. For instance, the mechanical behaviour of the fibre ring with respect to elongation under pressure stress and elongation, for example, as a result of torsional movements, bending movements or transverse displacement movements can already be different due to the type of fabric structure, depending on whether the fibre ring was woven, knitted, braided or manufactured in another manner. In this respect, among other things, the orientation of the fibres, of which the fibre ring is made up, in their slanting position with respect to the direction of the longitudinal axis of the artificial intervertebral disc play a roll for the forces to be transferred between the end plates. A conversion of the different stresses which occur into a compression of the disc can be achieved by the combination of the fibre ring and the nucleus and by an opposite slanting position of the fibres relative to the direction of the longitudinal axis of the artificial intervertebral disc.
The nucleus is filled as a fluid into the artificial intervertebral disc under pressure through a closable opening in one of the cover plates. A fluid in its widest sense can also be understood, for example, as a dried gel present in granular or powder form or as a substance of comparable consistence, with the fluid solidifying after a certain period of time into an elastic or at least partly plastic body or permanently maintaining its fluid properties as a more or less viscous liquid. It is decisive for the function of the artificial intervertebral disc that the nucleus formed in this way preferably completely fills up the hollow space which the cover plates form with the fibre ring, on the one hand, and has permanent low incompressibility and is under pressure bias, on the other hand. It is thereby ensured that the nucleus is in direct active connection with the fibre ring and can so act as a force transformer. If the artificial intervertebral disc built up in this manner is exposed, for example, to a pressure stress, the nucleus must escape in a partly radially outward manner under the compression effect and thereby expands the fibre ring, whereby the forces from the pressure stress are at least partly transformed into the fibre ring.
Since the nucleus is already under a certain pressure bias, which can be set as desired within wide limits by the filling pressure, in the unstressed state via the fibre ring, the starting stiffness can already be adjusted solely by the filling pressure of the fluid. The possibility thus exists to adapt the elastic properties to the individual needs of a patient while observing a pre-determined geometry for the artificial intervertebral disc and without changing or replacing the materials which make up the artificial intervertebral disc. In this manner, the non-linear stress characteristics can be matched, for example, to the body weight of the patient. Furthermore, the elastic behaviour can naturally be influenced both by the selection of the materials of which the nucleus or the fibre ring are respectively made up and by the specific design of the geometry of the components making up the artificial intervertebral disc.
The materials which make up the fibre ring include biocompatible plastics to the extent that they come into contact with body tissue. The fibre ring itself can be permeable for the body""s own liquids, but must be completely impermeable for the nucleus. The end plates can be made of a metal, for example of robust titanium, or of a robust titanium alloy, which is plastically deformable for the anchoring of the fibre tube. The outer surfaces of the end plates can have zones with a metal web which facilitate the growing in of bone material and thus the intergrowth of the adjoining vertebrae with the outer surfaces of the end plates. An improvement of the anchoring between the end plate and the vertebra with respect to lateral forces can be achieved by one or more projecting ribs which extend, for example, from ventral to dorsal. An additional toothed arrangement at the outer edge of the end plate, whichxe2x80x94like the projecting ribxe2x80x94does not have to be present mandatorily, can likewise substantially improve the connection between the end plate and the vertebra with respect to shear stresses. In this respect, the end plate does not necessarily have to be made up of metal, but suitable biocompatible plastics can, for example, also be considered as materials for the composition of the end plate.